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(CNN)A 3-year-old-boy who locked himself in a hot car and died in Texas is a tragedy that happens more often than people realize, a safety expert told CNN Friday.

On Thursday, the boy in Houston entered the car parked near a house to retrieve his toy from the back seat, said Kese Smith, spokesperson of the Houston Police Department. He entered through the unlocked front door and climbed into the back seat.

Once he got into the back seat, he couldn't open the back door because the child protective locks were on, Smith said.

Since 1998, more than 660 children across the United States have died from heatstroke when unattended in a vehicle, and 29% of those kids were playing in an unattended vehicle or gained access on their own, said Amy Artuso, a program safety manager for the National Safety Council.

Police are still investigating but the Houston death appears to be a tragic accident, Smith said. It was extremely hot in Houston on Thursday, with a high of 100 degrees.

Artuso said the death could have happened in practically any state in the summer.

"As a reminder, due to the way vehicles heat up -- like a greenhouse effect -- this tragedy can happen in any state, and cracking windows does not make a significant difference to decrease the internal temperature inside a vehicle," Artuso said.

Georgia man accused of murder in hot-car death

While no charges have been filed in the Houston case, a Georgia man is set to to go to trial soon in the case where prosecutors say they believe he used a hot car to kill his toddler son in metro Atlanta.

CNN-affiliatte WSB reported Thursday that a judge has picked the city of Brunswick in south Georgia as the location for the Ross Harris' murder trial,

Superior Court Judge Mary Staley granted a motion in May to move the trial out of Cobb County because of excessive publicity about the case in metro Atlanta